The present invention relates to an optical encoder for use in detecting an amount of movement of a movable portion of an apparatus.
It has been difficult to reduce the size and cost of a conventional optical encoder, since various optical components need to be assembled and adjusted. The applicant of the present invention, on the other hand, has developed an optical encoder which has a surface emitting semiconductor laser (hereinafter referred to as "surface emitting laser") as a light source and which permits reduction in size and cost.
FIG. 32 shows the structure of an optical encoder having the surface emitting laser developed by the applicant as a light source. In this optical encoder, when a beam has been radiated onto a scale 20 from a surface emitting laser 10, the beam reflected from or passing through the scale 20 is detected by a photodetector 30, 30'.
In general, the surface emitting laser has a smaller widening angle of an emission beam, which is determined by an emission window width (opening length) a, than a conventional edge-emitting semiconductor laser. Accordingly, the optical encoder developed by the applicant does not require a collimator lens or an index scale, which is needed in the conventional optical encoder. Thus, the size and cost of the optical encoder developed by the applicant can be reduced.
In the optical encoder using the conventional surface emitting laser, it is supposed that when a distance L between a light source and a scale is less than a predetermined distance (a.sup.2 /.lambda.), the size of the optical spot formed on the scale by the surface emitting laser is substantially equal to an opening width a in the scale pitch direction of the emission window of the surface emitting laser. Thus, there is no choice but to limit the value of the scale pitch p to p.gtoreq.a. In addition, since it is supposed in the prior art that the size of the optical spot formed on the scale is substantially equal to L.lambda./a when L&gt;a.sup.2 /.lambda., the scale pitch p is limited to p.gtoreq.L.lambda./a. It has not been considered to more decrease the value p.
On the other hand, an increase in resolution of the encoder has still been demanded in order to achieve a higher performance of a machine tool, a measuring device, etc. High resolution is indispensable for small-sized, low-cost optical encoders.